A Small-Block for the 21st Century
An Evolution With a Focus on Efficiency
Much consternation and speculation over the 2014 Chevrolet Corvette's power plant can now be put to rest. The C7 'Vette will be powered by a normally aspirated pushrod V8. The sun still rises in the east; the sky continues not to fall.
At a technical overview held this week at GM's Powertrain Engineering Center in Pontiac, Michigan, GM engineers lifted the skirt on the fifth-generation (Gen V) of their "small-block" V8 engine family, the first recipient of which will be the 2014 Chevrolet C7 Corvette. Say good-bye to the LS series — for the variant of the Gen V that will reside in the C7, GM resurrected the LT1 moniker. This will be the third go-around for the LT1 label, which first saw usage in 1970 (as "LT-1") and again in 1992, sans hyphen.
All in the Family
The Gen V eight-cylinder is an important engine family for GM. It will find a home in millions of trucks — the primary recipient of the engine in production volume terms — and cars for years to come. With stringent fuel economy standards on the horizon, efficiency took center stage in the development of the new engine.
Like the Gen IV, the Gen V is a pushrod, two-valve, 90-degree all-aluminum V8. However, the decision to incorporate direct injection in the Gen V engine set into motion a cascade effect that resulted in what is essentially an all-new power plant. Hardware-wise, the LT1 offshoot of Gen V shares only valve retainers, wrist pins and two starter bolts with the outgoing LS3 engine.
Yet the Gen V's fundamental architecture is heavily carried over from the Gen IV — bore spacing, crank-to-cam centerline distance, bottom-end bearing dimensions, deck height, connecting rod length, piston compression height, cylinder head bolt spacing and more are all carried over. So despite its newness, Gen V will be familiar to anyone steeped in the particulars of the Gen IV family.
Fully Exploiting Direct Injection
GM didn't just slap DI hardware onto the existing heads and call it a day. Remember, we're talking about a company that boasts some of the most comprehensive analytical capabilities of any automaker in the world. Instead, an exhaustive study of combustion chambers and port geometries was undertaken to determine which combination produced the best mix of charge preparation, efficiency, knock tolerance and power. Some 75 configurations were modeled and evaluated before settling on the one that went into the Gen V.
This effort is truly at the core of the pushrod engine family's new lease on life. GM brass confidently states that they'll put the Gen V's combustion system up against that of any overhead cam engine.
The resulting chamber geometry has numerous small yet critical differences from the Gen IV. Valves are more upright and canted slightly to aid flow and to help direct the charge toward the plug, while the plug itself has been moved closer to the center of the chamber for a more rapid burn.
Intake valve diameter has shrunk by 1mm, and the valve positions themselves are reversed in Gen V — the intake valves are where the exhaust valves used to be, and vice versa. An upright, twisting intake port imparts a very specific motion to the airflow that heavily influences the manner in which the combustion magic will unfold once the injector sprays, the valves are shut and the piston rises. Piston crowns, too, have the bizarre topography common to direct-injection engines.
Each feature is a part of a very carefully orchestrated ballet of chemical energy release. The aftermarket will have its hands full with these new heads.
Focus on Efficiency
The bore and stroke of the LT1 variant of the Gen V engine — and hence the resulting 6.2-liter displacement — are identical to the LS3. Compression has been bumped up to 11.5:1, thanks to the cooling effect provided by direct injection. Head bolt diameter is up by 1mm to better manage the new engine's higher cylinder peak pressures.
GM offered cylinder deactivation in the truck variants of the Gen IV engine, but this feature was not deployed in earlier Corvettes due to noise issues induced by four-cylinder mode. With the 2014 Corvette, such issues have been addressed, and thus the LT1 includes cylinder deactivation. This plays a crucial role in improving fuel economy during light-load operation — GM says the C7 'Vette will return better than 26 mpg on the highway. In a twist of irony, the LT1's large displacement affords the Corvette better fuel economy because it provides more frequent opportunity for four-cylinder mode than would be possible otherwise.
Likewise, the LT1 brings variable valve timing to the Corvette. The LT1's cam profile is virtually identical to that of the LS3, but now there's a phaser on the LT1 cam that allows its timing to be advanced or retarded. Intake and exhaust valve timing is necessarily slaved to one another with this system. GM evaluated a "cam-in-cam" VVT system (à la Dodge Viper) that would allow fully independent timing of intake and exhaust valve events plus the ability to modulate valve overlap, but a cost/benefit analysis ruled it out.
At the end of the LT1's cam is a tri-lobe feature that drives the direct-injection fuel pump, which is mounted beneath the intake manifold. In fact, all of the direct-injection hardware is tucked away out of sight. The LT1 is one clean-looking engine.
Two New Oil Supply Systems
In the bottom end, a forged steel crank swings powder-metal conrods made from a higher grade of steel than the LS3 rods. Nodular iron six-bolt mains caps replace the powder-metal caps in Gen IV. Two oil delivery systems were developed for the LT1, a conventional 6-quart wet-sump and an 11.5-quart dry-sump system. Both sump setups will be offered on the C7 and include a variable-output pump to reduce parasitic loss. It's not clear yet exactly how the dry-sump system will be offered, be it a stand-alone option or part of a track-focused option package.
One big challenge for direct-injected engines is preventing the intake valves from accumulating crud introduced by recirculated crankcase vapors. Since in DI engines there's no injector in the port to constantly wash the intake valve with a very effective solvent we call gasoline, deposits can grow unchecked. For the Gen V engine GM addressed this potential for fouling two ways — by improving the intake valves' stem seals and tripling the capability of the air-oil separation system. Those odd-looking boxy doohickeys on the Gen V's valve covers house baffles that yank the oil out of the crankcase vapors before it can reach the valves.
Efficiency Breeds Power
Preliminary peak power and torque figures for the LT1 are 450 horsepower at 6,000 rpm and 450 pound-feet at 4,500 rpm, numbers that will likely rise a hair while GM finishes up the engine's certification.
Don't be fooled by the LT1's apparently modest peak increases over the LS3. Low and mid-speed output see big gains — below 4,000 rpm, an addition of roughly 50 lb-ft is on tap in the LT1 over the LS3. The LT1 even out-torques the LS7 below 3,000 rpm. Yet the LT1 is said to be one of the most fuel-efficient 450-hp engines you can buy.
All that's left now is to drive the thing.